Concerns have been raised about the plans to investigate people without their knowledge as the Government introduces measures to prosecute the partners of benefit fraudsters.

Announcing the changes yesterday, Associate Social Development Minister Chester Borrows said a new offence would be created to allow the partner or spouse of a person wrongly collecting a benefit to be charged.

It would mean partners could face a fine of up to $5000 or 12 months in jail, as well as their share of the repayments.

Last year more than $20 million was lost because of relationship fraud, making up one-third of welfare fraud cases.

"Prosecuting partners who profit from welfare fraud will ensure that both parties who profit from the crime are punished, and will help the taxpayer recover the lost money faster," Mr Borrows said.

Other measures to be introduced include new powers for the Social Development Ministry to verify information provided by beneficiaries previously convicted of fraud and the removal of the requirement to tell people they were being investigated.

Beneficiary Advocacy Federation spokeswoman Kay Brereton said she was supportive of the move to target partners.

Currently the majority of people being punished for relationship fraud were women, who were often pressured into the crime. While there would be situations where a partner was not aware of the fraud, this would be the minority, she said.

"I'd love to think that this would discourage men, and I know I'm being very sexist but what we see is mostly men, to discourage them from pressuring their partner into doing this."

But she was dubious about plans not to tell beneficiaries they were being investigated for fraud.

There were legitimate situations where any problem could be cleared up simply by contacting the beneficiary, she said.

"We're very, very concerned about that, treating these particular individuals as guilty just because they're under investigation."

The new measures did not receive a warm reception from the opposition, with both Labour and the Greens criticising the announcement, and Government partners the Maori Party saying the emphasis was "all wrong".

Labour's social development spokeswoman, Jacinda Ardern, described the announcement as "pure politics" and said the Government should be focusing on tax evasion, which was a far bigger problem.

"This demonstrates an utter double-standard when it comes to recovering fraudulently obtained moneys."

Green Party co-leader Metiria Turei said it was clear the Government was pursuing its aggressive policy towards beneficiaries.